The present invention relates to focused ion beam (FIB) technology and more particularly to novel high brightness point ion sources using ionic compounds in the liquid state, including mixtures of molten salts, acids or bases. The novel high brightness point ion sources associate the desirable high brightness, ion intensities and energy distribution characteristics of conventional liquid metal ion sources (LMIS) with the possibility of significantly enlarging their spectrum of ion species. In particular, chemically reactive ions are now produced including positive and negative ions, molecular ions and even protons, which has not been made possible so far with said conventional LMIS sources. According to the present invention, any conventional focused ion beam system can be easily adapted to use such novel high brightness point ion sources.
Till today, almost all industrial point ion source applications are based upon liquid metal ion sources (LMIS), which gave rise in the 1970-1980 period to the spectacular explosion of focused ion beam (FIB) technology. In comparison to all other types of ion sources, LMIS sources exhibit quite excellent optical qualities (brightness and low energy spread) that allow the FIB systems incorporating the same to focus ionic spots of sub-micronic sizes with high current densities in the order of several A/cm2 that are well adapted to industrial applications. In particular, FIB systems are extensively used in the micro-electronic field, for the modification, reconfiguration, failure analysis and manufacturing of advanced semiconductor products, typically integrated circuits (ICs) and the surface analysis thereof. However, if LMIS sources are at the historical origin of the FIB technology expansion, they have inherent physical constraints which considerably limit the potential applications of FIB systems and scientific instruments derived therefrom. With LMIS sources, the ionizable source materials that can be used for ion generation are limited to a few pure metals and to some metal alloys. For reasons of reliability and of optical qualities, the most commonly used metal is gallium. Despite some recognized advantages, the use of LMIS sources in standard FIB systems exhibits some considerable limitations and inconveniences that are recited below.
First of all, there is no chemical reactivity effect associated with the collision process of sputtering (ionic bombardment) unlike in a conventional reactive ion etching process for example. This limits the removal rate of most of the most commonly used target materials to 1-4 sputtered atoms per incident ion at energies of about 30 kV. Because there is no by-product gas formation, the sputtered atoms are not evacuated during the process and a re-deposition of the sputtered materials near the attacked areas occurs. This undesired re-deposition is complicating many FIB based etching processes. In particular, if the redeposited material is of the conductive type it can create parasitical connections on the semiconductor product. Moreover, re-deposition of the sputtered material reduces the achievable sidewall angle, and thus results in aspect ratios of etched holes no greater than approximately 6:1. Etching a deep trench thus requires a lengthy process and can produce undesired damages to the hole neighboring areas.
Another consequence of this lack of chemical reactivity can be found in an analysis technique currently referred to as the SIMS (SIMS is an acronym for Secondary Ion Mass Spectrometry). This technique can be based on the use of gallium probes and has a recognized high local resolution (an important requirement of focused ion beam applications), so that such SIMS systems reach a local resolution of some tens of nanometers. However, one of the conditions for quantitative SIMS analysis is to have a high secondary ion emission yield. Because gallium doesn""t have any important effect of secondary ion yield stimulation, but has a high local resolution, the benefit of small sized gallium probes is balanced by the small emission rate of the secondary ions to be analyzed. On the contrary, commercially available SIMS systems using other ions which associate a chemical effect in addition to the collision effect of sputtering in order to enhance the secondary ion yield, unfortunately have a poor local resolution. In particular, cesium (Cs), which is one of the most chemically reactive metals, considerably enhances the secondary ion yield. Industrial SIMS systems may employ classical Cs sources (generally of the surface ionization type), but the brightness of these ion sources is low and thus cannot be compared with those of Ga LMIS sources. Moreover, the violent reactivity of cesium also makes its handling very difficult. Many attempts to produce Cs LMIS sources have been conducted in research laboratories, but because of the high chemical reactivity of cesium, Cs LMIS sources have never reached the acceptable reliability level that is required by SIMS systems used in the industry.
Finally, for example in applications such as quantum device fabrication, it is difficult to reduce the creation of ion beam-induced defects on the sample by a simple reduction of the ion beam energy, because this operation involves a very important loss in terms of current density, the machining time is significantly increased, and finally results in a costly process.
An attempt to solve the problem of the violent chemical reactivity of alkaline metals has been described in the article: Lithium ion emission from a liquid metal source of LiNO3, by A. E. Bell et al, published in the International Journal of Mass Spectrometry and Ion Processes, 88 (1989), pp 59-68. These authors conducted experiments with ion sources similar to LMIS sources. They substituted the alkaline metals with chemical compounds containing these metals. In particular, they produced a Li+ ion beam with a source using a pure molten binary salt, in this case the lithium nitrate (LiNO3). For these experiments they used a needle coated with this molten salt which was heated as standard and they called their ion sources xe2x80x9cliquid metal ion sources of LINO3xe2x80x9d, i.e. a variant of LMIS sources. They observed the generation of gas bubbles and very fast evaporation of the said molten salt. Measuring the energy distribution of the emitted ions with a retarding potential analyzer, they found a FWHM (full width at half maximum) energy spread of 110 eV which was comprised of two peaks. They concluded that this was due to the participation of gas-phase field ionization in addition to field desorption on the apex of the needle tip. Ion sources with such a large spread in energy distribution are unsuitable for use in industrial FIB systems because the focusing of the ion beam is roughly limited by chromatic aberrations.
On the other hand, in some advanced fields of micro-analysis, such as proton microscopy, ion beam lithography (no proximity effects for small structures and very high resist sensitivity to ions), localized Rutherford back-scattering analysis (RBS) and particle induced X-ray emission analysis (PIXE) with very high spatial resolution, the generation of protons (the known lightest ion) is quite impossible to obtain with FIB systems using conventional LMIS sources. As a matter of fact, if many attempts have been done to realize focused proton beam columns, these efforts have never ended in successful industrial applications. They often used field ionization sources in gas phase. In this particular case, the ion source generally consists of a tungsten needle that is cooled with liquid helium at a temperature of a few Kelvin degrees. A flux of hydrogen atoms is generated by field-ionization at the needle tip. These ion sources have a great theoretical brightness but imply sophisticated and thus expensive equipments. In addition, they have a very poor reliability and a low total ion current.
Consequently, it would be extremely interesting, especially for SIMS analysis of integrated circuits (ICs), but also for many other applications, to realize novel high brightness point ion sources for improved FIB systems. Such ion sources must retain the qualities of conventional LMIS sources in terms of pin-point emission characteristic and current density (brightness), but now offering a greater range of ion species, including but not limited to, alkaline metals and halogens. Finally, it would be also worthwhile to realize such novel high brightness point ion sources capable to work with ionizable source materials selected among liquid ionic compounds to benefit from the existence of pre-existing ions therein and to avoid the use of chemically very reactive and dangerous species such as Cs+ or Cxe2x88x92 ions. The present invention provides high brightness point ion sources adapted to work with liquid ionic compounds that meet all these requirements and thus present significant improvements and advantages in comparison with conventional LMIS sources.
According to the present invention, there is first described novel high brightness point ion sources that are adapted to operate with a wide range of ionizable source materials, that are ionic compounds in the liquid state, referred to hereinbelow as liquid ionic compounds. Typically, liquid ionic compounds in consideration include mixtures (eutectic or not) of molten salts, bases or acids. Generally, mixtures of salts require to be heated up to their melting point temperature to reach this liquid state, unlike bases and acids that are liquid at room temperature. A wide range of positive and negative ions of highly chemically reactive ion species (e.g. halogens, alkaline metal radicals and the like) and even protons become available. Because the ions are pre-existing in the liquid ionic compound, the ion emission mechanism onset only requires low extraction voltages (2,5 to 3,5 kV).
A typical novel high brightness point ion source of the present invention is basically comprised of two parts: the needle assembly and the extraction assembly. The needle assembly consists of a point shaped needle made of a refractory ceramic material, whose sharpened extremity is referred to as the tip. The needle is heated by a coil made of stainless steel or other suitable material which is tightly wound on a portion of the needle adjacent to the tip. The other extremity of the needle is lodged in a recess of an insulating support which is provided with two pins cast therein. Each extremity of the coil is welded to a pin. The pins are connected to a heating current supply. The extraction assembly is comprised of a circular extracting electrode provided with a central aperture that is screwed on a cylindrical body, so that the spacing between the tip apex and the center of the aperture is adjustable. The extraction electrode and the cylindrical body are both made of metal. The needle assembly is mounted inside the cylindrical body and accurately affixed thereto by centering screws. An extraction voltage supply applies a potential difference between the extraction assembly and the coil. Before usage, the needle assembly is coated with the liquid ionic compound, for instance by dipping in a crucible containing the liquid ionic compound. The whole surface of the needle is wetted with the said liquid ionic compound and a reservoir thereof is formed at the coil location. The needle assembly is then inserted in the extraction assembly and appropriately fixed therein. The high brightness point ion source is then ready for use in a FIB column. The present invention further encompasses a modified focused ion beam column for adaptation to the novel high brightness point ion sources. In particular, the modified focused ion beam column of the present invention is provided with a system adapted to apply reversible polarities to some or all electrical components of the FIB system (source voltages, electronic lenses, deflecting electrodes and the like) in order to work with ion beams of either positive or negative type.
Therefore, the novel high brightness point ion sources of the present invention not only retain most of the advantages of the conventional LMIS sources and in particular the same brightness, but add significant improvements thereto, so that they can be advantageously used in almost all industrial FIB applications known to date.
Therefore, it is a primary object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds which consists of mixtures, e.g. mixtures of molten salts, acids or bases.
It is another primary object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds wherein the emitting needle is made of a refractory ceramic material that can be heated either directly or indirectly.
It is another object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds which consists of mixtures of molten salts, acids or bases that have a low vapor pressure and a low melting point temperature.
It is another object of the present invention to provide novel high brightness point ion sources that are adapted to use liquid ionic compounds which preserves the desired high brightness, pin-point emission and low energy spread of standard LMIS sources.
It is another object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds that are adapted to produce a large spectrum of highly chemically reactive ion species including alkaline metals and halogens.
It is another object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds that have pre-existing ions for intense positive and negative ion emission.
It is another object of the present invention to provide novel high brightness point ion sources using liquid ionic compounds that are adapted to produce protons.
It is another object of the present invention to provide a process for generating ions in FIB columns with such novel high brightness point ion sources using liquid ionic compounds.
It is still another object of the present invention to provide improved FIB systems that are adapted to such novel high brightness point ion sources using liquid ionic compounds to produce beams of new ion species with high local resolution and spots of very sub-micronic sizes.
The novel features believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may be best understood by reference to the following detailed description of an illustrated preferred embodiment to be read in conjunction with the accompanying drawings.